264 Mr. T, Tate on a new Self-registering Mei'cury Barometer. 



that of the minimum pressure. A slit is made in the board A E, 

 covering the upper portion of the frame, to allow the tube E M 

 freely to ascend or descend, as the case may be. C, c, and d 

 are scales for reading off the height of the mercury column ba- 

 lancing the pressure of the atmosphere at any time. The upper 

 portion, K L, of the tube D L has a greater section than the lower 

 portion, and the section of the tube I F at the part I is greater 

 than that of the lower portion. The action of the instrument 

 is as follows. 



If I C be the column of mercury in the tube corresponding to 

 mean atmospheric pressure, then the instrument is so adjusted 

 that the lower extremity, D, of the tube shall be at, or near to, the 

 middle of the mercury cup I F, and that the liquid in the jar shall 

 stand at, or near to, H, the middle of the tube EM. In this 

 position (as well as in all other positions) of the instrument, the 

 weight of the displaced liquid will be equal to the weight of the 

 glass float added to the weight of mercury equal in volume to 

 the space occupied by the mercury in the cup. 



Wow supposing an augmentation to take place in the atmo- 

 spheric pressure, then the mercury will rise in the tube I C ; and 

 a portion of mercury being taken from the cup I F, the float will 

 rise ; and this will produce a further rise of the mercury in the 

 tube ; and this will go on until the loss of the force of floatation 

 is equal to the weight lost by the mercury in the cup. On the 

 other hand, when a reduction takes place in the atmospheric 

 pressure, the column I C will fall, and the mercury in the cup 

 being augmented, the float will sink, which will cause a further 

 fall in the column in the tube ; and this will go on until the in- 

 crease of the force of floatation is balanced by the gravity of the 

 mercury added to the cup. Hence it follows that the rise or 

 fall, as the case may be, of the mercury in the tube D L will be 

 greater than that which would be simply due to the change in 

 the height of the ordinary barometric column. 



The scales C, c, d are uniform in their graduations when the 

 external sections of the tubes E M and D G are uniform, together 

 with the internal section of the tube K L. These scales are gra- 

 duated in the following manner : — 



The actual height of the column I C is observed, and corre- 

 sponding marks are placed upon the different lines on which 

 the scales are to be formed ; after a change has taken place in 

 the atmospheric pressure, the height of the mercury column in 

 the tube I L is again observed ; and corresponding marks being 

 made on the different lines of scales as before, the intervals be- 

 tween these marks will have the same reading as the difference 

 between the two columns of observation : the scales may then 

 be extended according to the readings of these intervals. 



